1. Field of the Invention
The present invention relates to a continuous blood-pressure monitoring apparatus which includes a pressure-pulse-wave sensor adapted to be pressed against an artery of a living subject via the skin and continuously monitors blood pressure of the subject based on a pressure pulse wave detected by the sensor.
2. Related Art Statement
There is known a continuous blood-pressure monitoring apparatus which includes an inflatable cuff adapted to be worn on a portion of a living subject; a blood-pressure determining means for determining a blood pressure of the subject based on a signal obtained while a pressure in the cuff is slowly changed; a pressure-pulse-wave detecting device which includes a pressure-pulse-wave sensor adapted to be pressed against an artery of a prescribed portion of the subject and continuously detects a pressure pulse wave produced by the artery; a relationship determining means for determining, in advance, a relationship between blood pressure and magnitude of pressure pulse wave, based on the blood pressure determined by the blood-pressure determining means and a magnitude of the pressure pulse wave detected by the pressure-pulse-wave detecting device; and a blood-pressure monitoring means for successively determining, according to the thus determined relationship, a blood-pressure value of the subject based on each of respective magnitudes of the pressure pulse wave detected by the pressure-pulse-wave detecting device. Since the blood-pressure values successively determined by the continuous blood-pressure monitoring apparatus are very highly reliable, the apparatus can be used in those cases in which strict blood-pressure monitoring is needed. This apparatus is disclosed in, e.g., Japanese Utility Model Document No. 2-82309 or its corresponding U.S. Pat. No. 5,139,026.
In the continuous blood-pressure monitoring apparatus, disclosed in the above-indicated document, in which the pressure-pulse-wave sensor is pressed against an artery of a prescribed portion of a living subject, a condition under which the sensor is pressed against the artery may be changed by, e.g., a change of a state in which the sensor is worn, caused by a physical motion of the subject. Hence, in order to increase the reliability of blood-pressure values determined by the blood-pressure monitoring means, a calibration is periodically carried out to update the relationship between blood pressure and magnitude of pressure pulse wave. In each calibration, the blood-pressure determining means determines a new blood pressure of the subject in a process in which the pressure of the cuff is changed in a prescribed manner, and the relationship determining means determines a new relationship between blood pressure and magnitude of pressure pulse wave, based on the new blood pressure determined by the blood-pressure determining means and a magnitude of the pressure pulse wave detected by the pressure-pulse-wave sensor during the above-indicated process.
However, when each calibration is carried out, the cuff is inflated to press the subject""s portion, thereby causing the subject to feel discomfort. In addition, the calibration is periodically carried out irrespective of whether the condition under which the sensor is pressed is appropriate or not, and the calibration period needs to be shortened to increase the reliability of continuous monitoring of blood pressure. Thus, the burden exerted on the subject is increased.
In order to solve the above-indicated problem, Japanese Patent Document No. 7-284479 or its corresponding U.S. Pat. No. 5,860,932 discloses a continuous blood-pressure monitoring apparatus in which a pressure-pulse-wave sensor is worn on a portion of a subject that is located on a downstream side of a cuff, a pressure of the cuff is increased at a prescribed rate, and a judging means judges whether a relationship between blood pressure and pressure-pulse-wave magnitude is appropriate, based on a shape or an area of a pressure pulse wave detected by the sensor during the increasing of the cuff pressure. More specifically described, the disclosed apparatus determines, in a state in which the cuff pressure would be substantially equal to, or somewhat higher than, a diastolic blood pressure of the subject, the diastolic blood pressure of the subject by utilizing the fact that the tendency of change of respective shapes or areas of respective heartbeat-synchronous pulses of the pressure pulse wave, successively detected by the sensor during the increasing of the cuff pressure, significantly changes around the diastolic blood pressure, compares the thus determined diastolic blood pressure with a diastolic blood pressure determined based on a magnitude of a heartbeat-synchronous pulse of the pressure pulse wave according to the relationship between blood pressure and pressure-pulse-wave magnitude, and judges whether the relationship is appropriate. If it is judged that the relationship is appropriate, then it is not needed to carry out a calibration, which leads to reducing the discomfort the subject suffers.
However, even in the above-described continuous blood-pressure monitoring apparatus, the cuff pressure is increased up to a value substantially equal to, or somewhat higher than, the diastolic blood pressure, so as to judge whether the relationship is appropriate or not. Thus, the discomfort the subject suffers is not sufficiently reduced.
In addition, in the above-described continuous blood-pressure monitoring apparatus, the pressure-pulse-wave sensor needs to be worn on the subject""s portion located on the downstream side of the cuff. On the other hand, in many cases, the continuous blood-pressure monitoring apparatus is used during a surgical operation or in an intensive care unit when or where many devices are connected to the subject and, in some cases, the pressure-pulse-wave sensor cannot be worn on the subject""s portion located on the downstream side of the cuff.
It is therefore an object of the present invention to provide a continuous blood-pressure monitoring apparatus which sufficiently reduces discomfort felt by a living subject and sufficiently increases freedom of wearing.
The above object has been achieved by the present invention. According to a first aspect of the present invention, there is provided an apparatus for continuously monitoring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a portion of the subject, a cuff pulse wave including a plurality of heartbeat-synchronous pulses occurring to the cuff while a pressure in the cuff is changed; a blood-pressure determining means for determining a blood pressure of the subject based on a signal obtained while the pressure of the cuff is changed; a pressure-pulse-wave detecting device which includes a pressure-pulse-wave sensor that is adapted to be pressed against an artery of the subject and which continuously detects, through the pressure-pulse-wave sensor, a pressure pulse wave that is produced by the artery and includes a plurality of heartbeat-synchronous pulses; a relationship determining means for determining a relationship between blood pressure and magnitude of pressure pulse wave, based on the blood pressure determined by the blood-pressure determining means and a magnitude of the pressure pulse wave detected by the pressure-pulse-wave detecting device; a blood-pressure monitoring means for iteratively determining, according to the thus determined relationship, a monitor blood-pressure value of the subject based on a magnitude of each of the heartbeat-synchronous pulses of the pressure pulse wave detected by the pressure-pulse-wave detecting device; a pulse-wave-propagation-velocity-related-information obtaining means for iteratively obtaining, in a state in which the pressure of the cuff is held at a prescribed pulse-wave-detection pressure lower than a diastolic blood pressure of the subject, a piece of pulse-wave-propagation-velocity-related information which is related to a velocity at which a pulse wave propagates through the artery of the subject, based on a time of occurrence of a prescribed periodic point of a heartbeat-synchronous pulse of the cuff pulse wave and a time of occurrence of a prescribed periodic point of a corresponding heartbeat-synchronous pulse of the pressure pulse wave; a propagation-velocity-related-information-change-value determining means for periodically determining, at a prescribed judgment period, a change value of the pieces of pulse-wave-propagation-velocity-related information obtained by the pulse-wave-propagation-velocity-related-information obtaining means; a monitor-blood-pressure-change-value determining means for periodically determining, at the judgment period, a change value of the monitor blood-pressure values determined by the blood-pressure monitoring means; and a relationship checking means for comparing the change value of the pieces of pulse-wave-propagation-velocity-related information, determined by the propagation-velocity-related-information-change-value determining means, and the change value of the monitor blood-pressure values, determined by the monitor-blood-pressure-change-value determining means, with each other, and thereby judging whether the relationship between blood pressure and magnitude of pressure pulse wave, determined by the relationship determining means, is appropriate.
According to this aspect, the pulse-wave-propagation-velocity-related-information obtaining means obtains the piece of pulse-wave-propagation-velocity-related information, between the cuff and the pressure-pulse-wave detecting device, and the propagation-velocity-related-information-change-value determining means determines, at the judgment period, the change value of the piece of pulse-wave-propagation-velocity-related information. Since the pulse-wave-propagation-velocity-related information changes with the change of blood pressure of the subject, the change value of the piece of pulse-wave-propagation-velocity-related information corresponds to the amount of change of blood pressure. In addition, the change value of the monitor blood-pressure value determined by the monitor-blood-pressure-change-value determining means also corresponds to the amount of change of blood pressure. However, when the condition under which the pressure-pulse-wave detecting device is worn on the subject has changed and the monitor blood-pressure value determined by the blood-pressure monitoring means is not accurate, the change value of the monitor blood-pressure value largely differs from the change value of the piece of pulse-wave-propagation-velocity-related information. Therefore, the relationship checking means compares the change value of the monitor blood-pressure value and the change value of the piece of pulse-wave-propagation-velocity-related information, with each other, and judges whether the relationship between blood pressure and pressure pulse wave, determined by the relationship determining means, is appropriate.
Therefore, a longer period can be employed to operate the blood-pressure determining means and thereby update the relationship between blood pressure and pressure pulse wave, and accordingly the discomfort the subject feels can be reduced. In addition, since the pulse-wave-propagation-velocity-related information is obtained based on the cuff pulse wave detected in the state in which the pressure of the cuff is held at the prescribed pulse-wave-detection pressure lower than the diastolic blood pressure of the subject, the subject feels minimized discomfort only. Moreover, since the pulse-wave-propagation-velocity-related information can be determined even if the pressure-pulse-wave detecting device may not be worn on the downstream side of the cuff, the detecting device can be worn on the other arm than the arm around which the cuff is wound.
According to a second aspect of the present invention, there is provided an apparatus for continuously monitoring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a portion of the subject, a cuff pulse wave including a plurality of heartbeat-synchronous pulses occurring to the cuff while a pressure in the cuff is changed, a blood-pressure determining means for determining a blood pressure of the subject based on a signal obtained while the pressure of the cuff is changed; a pressure-pulse-wave detecting device which includes a pressure-pulse-wave sensor that is adapted to be pressed against an artery of the subject and which continuously detects, through the pressure-pulse-wave sensor, a pressure pulse wave that is produced by the artery and includes a plurality of heartbeat-synchronous pulses; a first relationship determining means for determining a first relationship between blood pressure and magnitude of pressure pulse wave, based on the blood pressure determined by the blood-pressure determining means and a magnitude of the pressure pulse wave detected by the pressure* pulse-wave detecting device; a blood-pressure monitoring means for successively determining, according to the thus determined first relationship, a monitor blood-pressure value of the subject based on a magnitude of each of the heartbeat-synchronous pulses of the pressure pulse wave detected by the pressure-pulse-wave detecting device; a standard-pulse-wave-propagation-velocity-related-information obtaining means for obtaining, as a standard piece of pulse-wave-propagation-velocity-related information, a piece of pulse-wave-propagation-velocity-related information which is related to a velocity at which a pulse wave propagates through the artery of the subject, based on a time of occurrence of a prescribed periodic point of a heartbeat-synchronous pulse of the cuff pulse wave in a first time duration comprising at least one of a first time period in which the pressure of the cuff is changed, a prescribed preceding time period preceding the first time period, and a prescribed following time period following the first time period, and a time of occurrence of a prescribed periodic point of a corresponding heartbeat-synchronous pulse of the pressure pulse wave in the first time duration; a second relationship determining means for determining a second relationship between blood pressure and pulse-wave-propagation-velocity-related information, based on the blood pressure determined by the blood-pressure determining means and the standard piece of pulse-wave-propagation-velocity-related information obtained by the standard-pulse-wave-propagation-velocity-related-information obtaining means; a cuff-pressure changing means for periodically increasing, at a prescribed judgment period, the pressure of the cuff up to a prescribed pulse-wave-detection pressure lower than a diastolic blood pressure of the subject; a judgment-pulse-wave-propagation-velocity-related information obtaining means for obtaining, as a judgment piece of pulse-wave-propagation-velocity-related information, a piece of pulse-wave-propagation-velocity-related information which is related to the velocity at which the pulse wave propagates through the artery of the subject, based on a time of occurrence of a prescribed periodic point of a heartbeat-synchronous pulse of the cuff pulse wave occurring to the cuff in a state in which the pressure of the cuff is held at the pulse-wave-detection pressure by the cuff-pressure changing means, and a time of occurrence of a prescribed periodic point of a corresponding heartbeat-synchronous pulse of the pressure pulse wave detected by the pressure-pulse-wave detecting device in the state; an estimated-blood-pressure determining means for determining, according to the second relationship, an estimated blood-pressure value of the subject based on the judgment piece of pulse-wave-propagation-velocity-related information obtained by the judgment-pulse-wave-propagation-velocity-related-information obtaining means; and a relationship checking means for comparing the estimated blood-pressure value determined by the estimated-blood-pressure determining means, and a monitor blood-pressure value determined by the blood-pressure monitoring means based on a magnitude of a heartbeat-synchronous pulse of the pressure pulse wave detected by the pressure-pulse-wave detecting device in a second time duration comprising at least one of a second time period in which the pressure of the cuff is held at the pulse-wave-detection pressure by the cuff-pressure changing means, a prescribed preceding time period preceding the second time period, and a prescribed following time period following the second time period, with each other, and thereby judging whether the relationship between blood pressure and magnitude of pressure pulse wave, determined by the relationship determining means, is appropriate.
According to this aspect, the estimated-blood-pressure determining means determines the estimated blood-pressure value based on the judgment-pulse-wave-propagation-velocity-related information according to the relationship between blood pressure and pulse-wave-propagation-velocity-related information. If the condition under which the pressure-pulse-wave detecting device is worn on the subject has changed and accordingly the monitor blood-pressure value determined by the blood-pressure monitoring means is not accurate, the monitor blood-pressure value largely differs from the estimated blood-pressure value. Hence, the relationship checking means compares the estimated blood-pressure value and the monitor blood-pressure value determined based on the pressure pulse wave by the blood-pressure monitoring means, with each other, and judges whether the relationship between blood pressure and pressure pulse wave, determined by the first relationship determining means, is appropriate or not.
Therefore, a longer period can be employed to operate the blood-pressure determining means and thereby update the relationship between blood pressure and pressure pulse wave, and accordingly the discomfort the subject feels can be reduced. In addition, since the judgment-pulse-wave-propagation-velocity-related information is obtained based on the cuff pulse wave detected in the state in which the pressure of the cuff is held by the cuff-pressure changing means at the value sufficiently lower than the diastolic blood pressure of the subject, the subject feels minimized discomfort only. Moreover, since the standard-pulse-wave-propagation-velocity-related information used to determine the relationship between blood pressure and pulse-wave-propagation-velocity-related information, and the judgment-pulse-wave-propagation-velocity-related information used to determine the estimated blood-pressure value can each be determined even if the pressure-pulse-wave detecting device may not be worn on the downstream side of the cuff, the detecting device can be worn on the other arm than the arm around which the cuff is wound.
According to a preferred feature of the first aspect of the invention, the relationship checking means judges that the relationship between blood pressure and magnitude of pressure pulse wave is not appropriate, when a relative value between the change value of the pieces of pulse-wave-propagation-velocity-related information, determined by the propagation-velocity-related-information-change-value determining means, and the change value of the monitor blood-pressure values, determined by the monitor-blood-pressure-change-value determining means, does not fall within a normal range, and the apparatus further comprises a normal-range determining means for determining, as the normal range, a narrower range when the monitor blood-pressure value used to determine the change value of the monitor blood-pressure values is lower than a prescribed danger value which indicates that the subject needs an urgent treatment, than a range determined thereby when the monitor blood-pressure value is not lower than the danger value.
According to this feature, when the monitor blood-pressure value is lower than the prescribed danger value, whether the relationship is appropriate or not is more strictly checked. Therefore, the accuracy of the monitor blood-pressure value determined when the blood pressure of the subject is low is improved, and whether the blood pressure of the subject is so low as to need an urgent treatment can be judged quickly and reliably.
According to a preferred feature of the second aspect of the invention, the relationship checking means judges that the relationship between blood pressure and magnitude of pressure pulse wave is not appropriate, when a relative value between the estimated blood-pressure value determined by the estimated-blood-pressure determining means, and a monitor blood-pressure value determined by the blood-pressure monitoring means based on a magnitude of a heartbeat-synchronous pulse of the pressure pulse wave detected by the pressure-pulse-wave detecting device in the second time period in which the pressure of the cuff is held at the pulse-wave-detection pressure by the cuff-pressure changing means, does not fall within a normal range, and the apparatus further comprises a normal-range determining means for determining, as the normal range, a narrower range when at least one of the estimated blood-pressure value and the monitor blood-pressure value is lower than a prescribed danger value which indicates that the subject needs an urgent treatment, than a range determined thereby when each of the estimated blood-pressure value and the monitor blood-pressure value is not lower than the danger value.
According to this feature, when at least one of the estimated blood-pressure value and the monitor blood-pressure value is lower than the prescribed danger value, whether the first relationship is appropriate or not is more strictly checked. Therefore, the accuracy of the monitor blood-pressure value determined when the blood pressure of the subject is low is improved, and whether the blood pressure of the subject is so low as to need an urgent treatment can be judged quickly and reliably.
Preferably, the relationship checking means comprises means for operating, when it is judged that the relationship between blood pressure and magnitude of pressure pulse wave is not appropriate, the relationship determining means to update the relationship.
According to this feature, since the inappropriate relationship is quickly updated and corrected and the monitor blood-pressure values are determined according to the corrected relationship, the reliability of the monitor blood-pressure values is improved.